Techniques of Forming Ohmic Contacts on GaN Light Emitting Diodes

ABSTRACT

A method of forming ohmic contacts on a light emitting diode that features a surface treatment of a substrate includes exposing a surface of a p-type gallium nitride layer to an acid-containing solution and a buffered oxide etch process. A quantum well is formed in a gallium nitride substrate and a layer of p-type gallium nitride is deposited over the quantum well. The surface of the p-type gallium nitride is exposed to an acid-containing solution and then a buffered oxide etch process is performed to provide an etched surface. A metal stack including a layer of silver disposed between layers of platinum is then deposited.

BACKGROUND OF THE INVENTION

The invention is directed to light emitting diodes and in particular toohmic contacts for light emitting diodes.

One technique for measuring the efficiency of light emitting diodes(LEDs) is by determining the luminance per watt. The luminance providedby light emitting diodes is dependent upon several factors such asinternal quantum efficiencies, as in the case of an injected carrier notbeing converted to a photon, or extraction efficiency, as in the case ofa small fraction of photons being successfully extracted from the lightemitting diode as opposed to being lost to internal absorption. Torealize high efficiency LEDS, both of these issues need to be addressed.The potential gain from improving extraction efficiency, however, islikely to be greater and simpler to accomplish than the gains fromimproving internal efficiency.

One technique to improve light extraction of visible light nitride LEDs,such as Gallium Nitride (GaN) LEDs, is achieved through use of highreflectivity metallurgies which are typically mounted to one side of theLED. GaN based devices typically require ohmic contact formation as ameans of establishing electrical contact to the device with minimalimpact on the operating voltage of the device. Thus, the highreflectivity metallurgies are typically employed in the ohmic contactand attached to a p-type GaN layer of the LED. One common approach is touse a silver containing layer in the ohmic contact. Silver is desirable,because of its high reflectance. The difference in the work functionbetween silver and the other materials from which the LED is fabricatedhas been problematic. For example, it is widely accepted that metalswith high work functions form the best contacts for p-type semiconductormaterials, while metals with low work functions form the best contactsfor n-type semiconductor materials. However, surface contamination ofthe metal semiconductor interface may degrade the ohmic contactperformance of metals. Contamination layers at the interface may producean unforeseen electronic state that may degrade the efficiency of theLED.

There is a need, therefore, to provide improve ohmic contact techniquesfor LEDs.

BRIEF SUMMARY OF THE INVENTION

This invention is directed to a method of forming ohmic contacts on alight emitting diode that features a surface treatment of a substratethat includes exposing a surface of a layer p-type gallium nitride to anacid-containing solution and a buffered oxide etch process. To that end,the method includes forming a quantum well in a gallium nitridesubstrate, depositing a layer of p-type gallium nitride upon saidquantum wells, exposing a surface of said p-type gallium nitride to anacid-containing solution, forming a cleaned surface; subjecting saidcleaned surface to a buffered oxide etch process, forming an etchedsurface; and generating, upon said etched surface, a metal stackincluding a layer of silver disposed between layers of platinum. Theseand other embodiments are discussed further below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a single light emitting diodemanufactured in accordance with the present invention;

FIG. 2 is a flow diagram showing the steps used to manufacture the lightemitting diode shown in FIG. 1;

FIG. 3 is a cross-sectional view showing a substrate upon which thelight emitting diode shown in FIG. 1 is produced;

FIG. 4 is a cross-sectional view showing the substrate in FIG. 3 with ap-type gallium arsenide layer disposed thereon;

FIG. 5 is a simplified cross-sectional view of the substrate shown inFIG. 4 having a patterned photo resist layer deposited thereon;

FIG. 6 is a simplified cross-sectional view of the substrate shown inFIG. 4 having a plurality of metal layers deposited thereupon;

FIG. 7 is a cross-section view of the substrate shown in FIG. 6 afterbeing subjected to a lift-off process that removed the photo resist; and

FIG. 8 is a simplified cross-sectional view of the substrate shown inFIG. 6, scored-regions formed thereon.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, shown is a light emitting diode 10 manufactured inaccordance with the present invention that includes a substrate 12formed of n-type gallium nitride GaN. An active layer 14 is formed uponsubstrate. Active layer 14 may comprise a single quantum well ormultiple quantum wells, with 2-10 quantum wells. A layer of p-typegallium nitride 16 is formed upon quantum wells 14. A metal stack 18 ispositioned upon layer 16 and is comprised of three separate metallayers, shown as 20, 22 and 24. Layers 20 and 24 are formed fromplatinum and layer 22 is formed from silver.

Substrate 12 may have a large-surface orientation within ten degrees,within five degrees, within two degrees, within one degree, within 0.5degree, or within 0.2 degree of (0 0 0 1), (0 0 0 −1), {1 −1 0 0}, {1 1−2 0}, {1 −1 0.+−.1}, {1 −1 0.+−.2}, {1 −1 0.+−.3}, {2 0 −2.+−.1}, or {11 −2.+−.2}. In one specific embodiment, the substrate has a semipolarlarge-surface orientation, which may be designated by (hkil)Bravais-Miller indices, where i=−(h+k), l is nonzero and at least one ofh and k are nonzero. The substrate may have a dislocation density below10.sup.4 cm.sup.-2, below 10³ cm⁻², or below 10² cm⁻². Substrate 12 mayhave an optical absorption coefficient below 100 cm10⁻¹, below 50 cm⁻¹or below 5 cm⁻¹ at wavelengths between about 465 nm and about 700 nm.The nitride base crystal may have an optical absorption coefficientbelow 100 cm⁻¹, below 50 cm⁻¹ or below 5 cm⁻¹ at wavelengths betweenabout 700 nm and about 3077 nm and at wavelengths between about 3333 nmand about 6667 nm. The surface of substrate 12 may have a dislocationdensity below 10⁵ cm⁻² and is substantially free of low-angle grainboundaries, or tilt boundaries, over a length scale of at least 3millimeters. Substrate 12 may be doped with any suitable n-type dopantsfrom group VI and group IV atoms, e.g., sulfur, selenium, tellurium,silicon, germanium. In the present embodiment, substrate 12 is dopedwith Si and O to dope our GaN, providing a dopant concentration ofapproximately of 3 E18 cm-3.

Active layer 14 may comprise of InGaN wells and GaN barrier layers. Inother embodiments, the well layers and barrier layers compriseAl_(w)In_(x)Ga_(1-w-x)N and Al_(y)In_(z)Ga_(1-y-z)N, respectively, where0≦w, x, y, z, w+x, y+z≦1, where w<u, y and/or x>v, z so that the bandgapof the well layer(s) is less than that of the barrier layer(s) and then-type substrate. The well layers and barrier layers may each have athickness between about 1 nm and about 20 nm. In another embodiment,active layer 14 comprises a double heterostructure, with an InGaN orAl_(w)In_(x)Ga_(1-w-x)N and Al_(y)In_(z)Ga_(1-y-z)N layer about 20 nm toabout 500 nm thick surrounded by GaN or Al_(y)In_(z)Ga_(1-y-z)N layers,where w<u, y and/or x>v, z. The composition and structure of the activelayer are chosen to provide light emission at a preselected wavelength.Active layer 14 may be left undoped (or unintentionally doped) or may bedoped n-type or p-type. Active layer 14 is formed upon substrate 12using standard processing techniques.

Layer 16 may be doped with any suitable p-type dopant, such as thosefrom group II or IV atoms, e.g., magnesium, zinc, cadmium, silicon,germanium. In the present example, layer is doped with magnesium toprovide a dopant concentration of approximately 1e20 cm⁻³.

Referring to FIGS. 2 and 3, substrate 12 is doped with n-type dopantsusing well known techniques, at step 100. At step 102, active layer 14is formed upon substrate 12 using well known techniques. Followingformation of active layer 14, p-type gallium nitride layer 16 is formedthereupon, shown in FIG. 4, at step 104 of FIG. 2. At step 106 surfaceof layer is exposed to an acid-containing cleaning solution. Thecleaning solution consists essentially of 15% of nitric acid by weight,27% of hydrochloric acid by weight and 58% of water by weight. Thisprovides cleaned surface 26.

Referring to both FIG. 2 and FIG. 5, at step 108 a patterned photoresist layer 28 is formed upon cleaned surface 26. Layer 28 has a shapeof a battlement leaving portions 30 of cleaned surface 26, with segments32 of photo resist material being present between adjacent portions 30.Following formation of patterned photo resist layer 28, substrate 12regions 30 and segments 32 are exposed to a buffered oxide etch process,at step 110. To that end, substrate 12 dipped into a solution consistingessentially of 2% hydrofluoric acid by weight and 8.75% ammoniumfluoride by weight, with the rest being water. At step 112, three metallayers are sequentially deposited upon portions and segments 32.Specifically, a platinum layer 34 is deposited, followed by depositionof a silver layer 36. Another platinum layer 38 is deposited upon silverlayer 36.

Referring to FIG. 2, at step 114 a lift-off process is undertaken toremove segments 38 and the portions of layers 34, 36 and 38 insuperimposition therewith, leaving a plurality of spaced-apart metalstacks 40. As a result, regions 42, shown in FIG. 7, of exposedsubstrate 12 remain between adjacent metal stacks 40.

Referring to both FIGS. 2 and 8, at step 116, a recess 44 is formed inregions 42, using desired techniques, such as laser etching. Recesses 44compromise the structural integrity of substrate 42 so that the same maybe segmented to produce light emitted diode 10, shown in FIG. 1.

It should be understood that the description recited above is an exampleof the invention and that modifications and changes to the examples maybe undertaken which are within the scope of the claimed invention.Therefore, the scope of the appended claims should be accorded thebroadest interpretation so as to encompass all such modifications andsimilar arrangements, including a full scope of equivalents.

1. A method of forming ohmic contacts on a light emitting diodecomprising: forming an active layer on a gallium nitride substrate;depositing a layer of p-type gallium nitride upon the active layer;exposing a surface of said p-type gallium nitride to an acid-containingsolution, forming a cleaned surface; subjecting the cleaned surface to abuffered oxide etch process, forming an etched surface; and generating,upon the etched surface, a metal stack including a layer of silver. 2.The method as recited in claim 1 wherein exposing further includesproviding nitric acid, hydrochloric acid and water in saidacid-containing solution.
 3. The method as recited in claim 1 whereinexposing further includes providing nitric acid, hydrochloric acid andwater in said acid containing solution, with said acid-containingsolution consisting essentially of 15% of said nitric acid by weight,27% of said hydrochloric acid by weight and 58% of said water by weight.4. The method as recited in claim 1 wherein subjecting further includesproviding said buffer oxide etch process with components consistingessentially of 2% hydrofluoric acid by weight and 8.75% ammoniumfluoride by weight.
 5. The method as recited in claim 1 whereingenerating further includes depositing said layer of silver between twolayers of platinum.
 6. The method as recited in claim 1 furtherincluding producing a patterned photo resist on said surface followed bydeposition of said metal stack.
 7. The method as recited in claim 1further including producing a patterned photo resist by covering asub-portion of said cleaned surface with remaining regions of saidcleaned surface being exposed followed by exposing said surface to saidbuffered oxide etch process.
 8. The method as recited in claim 1 furtherincluding producing a patterned photo resist on said surface followed bydepositing said metal stack on said patterned photo resist followingexposing said cleaned surface to said buffered oxide etch process. 9.The method as recited in claim 1 further including producing a patternedphoto resist on said surface following deposition of said metal stack onsaid patterned photo resist followed by exposing said surface to saidbuffered oxide etch process and lifting-off said photo-resist to producea plurality of spaced-apart metal stacks upon said substrate.
 10. Themethod as recited in claim 1 wherein depositing further includesproviding said layer with a p-type dopant concentration of 1e20 cm⁻³.11. A method of forming ohmic contacts on a light emitting diode, saidmethod comprising: forming a plurality of quantum wells on a galliumnitride substrate; depositing a layer of p-type gallium nitride uponsaid quantum wells; exposing a surface of said p-type gallium nitride toan acid-containing solution, forming a cleaned surface; producing apatterned photo resist on said cleaned surface covering sub-portionsthereof with remaining regions being exposed; subjecting said remainingregions to a buffered oxide etch process, forming etched regions;generating a metal stack on said etched regions; lifting-off saidphoto-resist to produce a plurality of spaced-apart metal stacks uponsaid substrate; and segmenting said substrate into a plurality ofdiscrete segments, each of which includes one of said plurality of metalstacks.
 12. The method as recited in claim 11 wherein depositing furtherincludes providing said layer with a p-type dopant concentration of 1e20cm⁻³.
 13. The method as recited in claim 11 wherein exposing furtherincludes providing nitric acid, hydrochloric acid and water in saidacid-containing solution.
 14. The method as recited in claim 11 whereinexposing further includes providing nitric acid, hydrochloric acid andwater in said acid containing solution, with said acid-containingsolution consisting essentially of 15% of said nitric acid by weight,27% of said hydrochloric acid by weight and 58% of said water by weight.15. The method as recited in claim 11 wherein subjecting furtherincludes providing said buffer oxide etch process with componentsconsisting essentially of 2% hydrofluoric acid by weight and 8.75%ammonium fluoride by weight.
 16. The method as recited in claim 11wherein generating further includes depositing a layer of silver betweentwo layers of platinum.
 17. A method of forming ohmic contacts on alight emitting diode, said method comprising: forming a plurality ofquantum wells on a gallium nitride substrate; depositing a layer ofgallium nitride upon said quantum wells with a p-type dopant at aconcentration of 1e20 cm⁻³; exposing a surface of said p-type galliumnitride to an acid-containing solution, consisting essentially of nitricacid, hydrochloric acid and water, forming a cleaned surface; producinga patterned photo resist on said cleaned surface exposing portionsthereof; subjecting said portions to a buffered oxide etch process,forming etched regions; depositing a metal stack on said etched regions;lifting-off said photo-resist to produce a plurality of spaced-apartmetal stacks upon said substrate; and segmenting said substrate into aplurality of discrete segments, each of which includes one of saidplurality of metal stacks.
 18. The method as recited in claim 17 whereinexposing further includes providing said acid-containing solution with15% of said nitric acid by weight, 27% of said hydrochloric acid byweight and 58% of said water by weight.
 19. The method as recited inclaim 17 wherein subjecting further includes providing said buffer oxideetch process with components consisting essentially of 2% hydrofluoricacid by weight and 8.75% ammonium fluoride by weight.
 20. The method asrecited in claim 17 wherein generating further includes depositing alayer of silver between two layers of platinum.